Optoelectronic device and process for acquiring symbols, such as bar codes, using a two-dimensional sensor

ABSTRACT

The invention relates to a device and a process for acquiring bichromatic bar codes, with a two-dimensional sensor with electronic scanning. The height Hy of the scanned portion is modified between at least two successive scanning operations. Thus, the device is or may be adapted to the type and/or to characteristics not known in advance of the code to be read.

TECHNICAL FIELD

This invention is generally related to machine-readable symbol readers.

BACKGROUND

In known optoelectronic devices for acquiring machine-readable symbols,such as bar codes, a diaphragm has a circular aperture of small diameterso as to prevent defocusing of an image and/or to increase the depth offield of the device. The small diameter of the aperture, however,reduces the intensity of reflected light received at the sensor and, inpractice, makes it necessary to use light sources having a high luminousintensity in order to compensate for the reduction in luminous intensityintroduced by the aperture. However, high intensity light sources areexpensive and lead to high power consumption.

While increasing the diameter of the aperture of the diaphragm increasesthe quantity of light received by the sensor, the increase in diameteralso reduces the depth of field of the device, thereby reducing theoverall efficiency of the device.

One attempt at solving these problems involves producing anoptoelectronic device as described in patent application EP-61000, wherethe diaphragm has an aperture having an asymmetrical elongation along anaxis orthogonal to the axis of the bar code, such as an aperture ofrectangular, rhombic or elliptical shape. This effectively increases thesensitivity of optoelectronic devices, which is proportional to theratio of collected flux to reflected flux. As a result, the depth offield of these devices may be increased without significantly affectingthe intensity collected on the sensor, thereby increasing the efficiencyof these devices. The relatively large dimensions of the diaphragmaperture, however, makes it necessary to use an asymmetrical diaphragm,and optical means for forming the image on the sensor having dimensionsgreater than those of conventional optical means, which increasesproduction costs and complexity greater than those of conventionaldiaphragms and optical means.

Another attempt at solving these problems involves producingoptoelectronic devices as described in International Patent ApplicationsWO-9620454 and WO-9847377, where the optical means are adapted toobtain, in the plane (XOZ) parallel to the optical plane, amagnification m₁ greater than the magnification m₂ in the plane (YOZ)perpendicular to the optical plane.

This approach, which can also be associated with that described in thepatent EP-61000, leads to an increase, along axes parallel to the barsof the bar codes, in the size of the illumination surface of the barcodes whose image is reflected on the sensor, and therefore to anincrease in the sensitivity of the optoelectronic device. It should benoted, furthermore, that since this increase in the sensitivity of thedevice results from the mere design of the optical means and not fromthe dimensions of the diaphragm aperture, a device of this type may beequipped with a conventional diaphragm with a circular aperture of smalldimensions and therefore with low-cost optical means of conventionaldimensions which is easy to produce.

With all these devices in which the diaphragm and/or the optical meansdo not form a symmetrical system generated by revolution round theoptical axis, the improvement in sensitivity is effective only when theoptical plane (plane containing the optical axis and the scanningdirection) coincides exactly with the nominal direction of reading ofthe bar code (perpendicular to the code bars and spaces). Now, as thebar code and/or the optoelectronic device in practice have unfixedorientations in space, this condition is rarely fulfilled. Thus, adevice of this type is extremely sensitive to alignment errors betweenthe optical plane and the normal direction of reading and is thereforedifficult to handle.

More generally, known optoelectronic devices for acquiringmachine-readable symbols can be configured for predeterminedcharacteristics of the symbols to be acquired and/or for predeterminedpositioning relative to the device. However, these optoelectronicdevices have inferior performance if the symbol does not have theseexpected characteristics or if the positioning is not perfect. As aresult, they suffer from a significant reading failure rate, inparticular in the case of plurimonodimensional symbols such as PDF 417codes.

U.S. Pat. No. 5,654,533 describes a two-dimensional symbol readercomprising a two-dimensional sensor and an automatic diaphragm of whichthe diameter varies to allow appropriate illumination of the sensor.This device does not attempt, and cannot solve the above mentionedproblem since, with this device, correct illumination of the sensorcorresponds to a diaphragm that produces an inadequate depth of field.Furthermore, this device is limited to the acquisition of bi-dimensionalsymbols by imagery, in other words by obtaining and analyzingtwo-dimensional images.

WO-98.16896 describes a two-dimensional symbol reader comprising both anelectronic scanning device having a two-dimensional sensor and a laserscanner device. This mixed reader enables the user to select one of thetwo devices depending on the symbol to be read. It is however verycomplex and therefore expensive, fragile and awkward to use. Inparticular, the embodiments disclosed herein avoid the use of laserdevices incorporating moving parts.

At present, therefore, there is no optoelectronic device for acquiringmachine-readable symbols, such as bar codes, with electronic scanningwhich has satisfactory performance, particularly in depth of field,which allows the acquisition of symbols with any characteristics whichmay be not be known in advance. For example, bar dimensions, barcontrast, type of codes, monodimensional or plurimonodimensional codes(in other words formed by a plurality of monodimensional bar codes) suchas the PDF 417 codes, or two-dimensional codes, etc.

SUMMARY

In one aspect, an optoelectronic device is capable of acquiringbichromatic machine-readable symbols, such as bar codes, formed frommonochromatic elements of geometric patterns (e.g., bars, squares,hexagons) having one of two levels of contrasting colors of which theshapes and disposition are adapted so that each code is able torepresent bi-uniquely a value of information to be acquired.

In another aspect, a process allows an optoelectronic device to acquiremachine-readable symbols based on symbol characteristics. In one aspect,a device and a process acquires machine-readable symbols havingdifferent characteristics, in particular of different types, and whichmay be adapted at the moment of acquisition, in particularautomatically, to the characteristics, in particular to the type ofsymbol to be acquired.

In another aspect, an optoelectronic device and process acquiresmachine-readable symbols with electronic scanning, while simultaneouslyproviding large depth of field and low rotational sensitivity toalignment errors between the optical plane and the nominal direction ofreading of the symbol, without necessitating the use of high intensitylight sources.

In yet another aspect, an optoelectronic device does not require a highdegree of precision in positioning of the machine-readable symbols to beacquired relative to the device in the relative spacing and rotationalalignment around the optical axis, and allow manual acquisition (inother words by relative manual positioning of the device and/or thesymbol) of the symbols.

In a further aspect, an optoelectronic device and a process manuallyacquires (by manual relative positioning of the symbol and/oroptoelectronic device) machine-readable symbols from relatively newsymbologies such as PDF 417 codes.

In yet a further aspect, an optoelectronic device can provide the abovebenefits while being inexpensively manufactured in a traditional manner,which requires no moving parts.

In still a further aspect, acquiring symbols can be performed by asimple and quick process, which can be entirely automated.

To this end, a non-limiting, illustrated embodiment of an optoelectronicdevice for acquiring bichromatic bar codes, comprises:

-   -   a reading window,    -   sensor means with electronic scanning comprising a        two-dimensional sensor comprising a plurality of individual        detectors known as pixels transmitting electrical signals        representing the quantity of light which they receive, the        sensor means being adapted to carry out electronic scanning or        at least a portion, known as scanned portion, of this        two-dimensional sensor in a direction, known as scanning        direction XX′, the pixels of the two-dimensional sensor being        ordered in a plurality of h rows juxtaposed in a direction,        known as direction YY′, perpendicular to the scanning direction        XX′, the two-dimensional sensor/extending in the direction YY′        over a height greater than a pixel, the scanned portion having a        dimension in the direction YY′, known as height Hy, which is        constant during each scanning operation, from one side to the        other of the two-dimensional sensor in the scanning direction        XX′,    -   optical means adapted to form, at least on the scanned portion        of the two-dimensional sensor, an image of a symbol or code to        be acquired located opposite the reading window, wherein, in        order to acquire a code placed opposite the reading window, the        sensor means are adapted to carry out at least two scanning        operations (i.e., passes) and to modify, between at least two        successive scanning operations, the height Hy of the scanned        portion of the two-dimensional sensor.

Throughout the text, the term “row” denotes each series of successiveindividual pixels which can be covered pixel by pixel during a scanningoperation in the scanning direction. A row is therefore defined by thegeometric arrangement of the pixels of the sensor in the scanningdirection XX′ and by the way in which these pixels consideredindividually are covered during the scanning operation. In the simplestcase of pixels arranged in lines and scanning carried out over eachline, a row corresponds to a line. However, if the pixels of twoadjacent lines are alternated during the scanning operation, a row isthus formed by the pixels of these two lines. While scanning along a rowin the scanning direction XX′, electrical signals are received from oneor more pixels arranged across the rows with respect to one another inthe direction YY′, perpendicular to the scanning direction XX′.

Also to this end, a non-limiting, illustrated embodiment of a method ofoperating an optoelectronic device for acquiring bichromatic symbols,comprises:

-   -   a reading window,    -   sensor means with electronic scanning in a global scanning        direction, known as scanning direction XX′ comprising a        plurality of individual light detectors known as pixels        transmitting electrical signals representing the quantity of        light which they receive, these sensor means comprising a        two-dimensional sensor of which the pixels are ordered in a        plurality of h rows juxtaposed in a direction, known as        direction YY′, perpendicular to the scanning direction XX′, this        two-dimensional sensor extending perpendicularly to the scanning        direction XX′ over a height greater than a pixel, the sensor        means being adapted to carry out electronic scanning of at least        a portion, known as scanned portion, of the two-dimensional        sensor having a dimension in the direction YY′, known as height        Hy, which is constant during each scanning operation, from one        side to the other of the two-dimensional sensor in the scanning        direction XX′,    -   optical means adapted to form, on the sensor means, an image of        a symbol or code to be acquired located opposite the reading        window,        a process for acquiring bichromatic bar codes, wherein, in order        to acquire a symbol or code placed opposite the reading window,        at least two scanning operations are carried out and, between at        least two successive scanning operations, the height Hy of the        scanned portion of the two-dimensional sensor is modified.

In a device and a process according to the invention, the height Hy canbe modified once; or several times but not between the scanningoperations each time; or between two successive scanning operations eachtime in order to acquire the same symbol or code.

To modify the height Hy of the scanned portion, it is possible to modifyeither the height of at least one row of the scanned portion (byselecting a row of which the pixels have a different height pyj) or thenumber of rows in this scanned portion, in other words the number ofsuccessive pixels in the direction YY′ of which the signals are added upin a same signal used during the decoding operation. These twovariations may be combined. It is in fact possible to modify both thenumber of rows and the height of at least one row. In fact, the heightHy of the scanned portion is equal to the sum of heights pyj of each rowj of this scanned portion. If all the heights pyj of the rows are equalto a same value py and if the scanned portion comprises by rows, theheight Hy of this scanned portion is equal to hy×py. If the rows do notall have the same height pyj,

${Hy} = {\sum\limits_{j = 1}^{hy}\;{pyj}}$

In a variation, therefore, the device according to the invention ischaracterized in that each row is formed by pixels all having the samedimension in this row in direction YY′, known as height pyj, wherein thepixel height pyj of at least one row of the two-dimensional sensor isdifferent from that of the pixels of at least one other row of thetwo-dimensional sensor and wherein, in order to modify the height Hy ofthe scanned portion, the sensor means are adapted to carry out at leastone scanning operation, known as first scanning operation, with at leastone row of pixels and at least one further scanning operation, known assecond scanning operation, with at least one row having a pixel heightpyj different from that of at least one row of the first scanningoperation. Advantageously and according to the invention, the sensormeans are adapted to carry out at least one second scanning operationwith at least one row having a pixel height pyj different from that ofeach row of at least one first scanning operation.

In a further variation of the invention, in order to modify the heightHy of the scanned portion, the sensor means are adapted to modify thenumber, known as pitch try, of successive rows of the scanned portion.

The scanned portion of the sensor is the one comprising the pixels ofwhich the signals are used to decode a symbol or code on the basis of ascanning operation. By modifying the value of the height Hy or theportion scanned between at least two scanning operations, the deviceadapts itself or may be adapted to the type and/or to thecharacteristics (which may be unknown) of the symbol or code to beacquired since at least some of the different values used for the heightHy will be most suitable.

In a first variation of the invention, the various possible values ofthe height Hy may be predetermined in advance (for example if thevariations in height Hy are obtained by selecting rows from a pluralityof different heights pyj) and optionally stored (for example variouspredetermined values for the number hy of rows) in the device comprisingelectronic processing means adapted subsequently to select the bestresult obtained by the various scanning operations in order to execute adecoding protocol. In particular, this variation is applicable if thetype of symbol is known but not the optical characteristics of the codesto be read (contrast, dimensions, etc.).

In a second preferred variation of the invention, the deviceautomatically adapts itself to the codes to be read of which the typeand characteristics may be unknown. Advantageously, the device accordingto the invention comprising electronic processing means adapted, duringeach reading of a symbol or code to be acquired:

-   -   to control the scanning operations by the sensor in the scanning        direction XX′ and receive the electrical signals issuing from        the pixels,    -   to execute a predetermined decoding protocol in order to obtain        the value of information represented by the symbol or code,        wherein the sensor means are adapted to, after each scanning        operation, execute treatment to optimize the height Hy in order        to improve the results of the subsequent scanning stage and        reduce the number of scanning stages required for decoding,        wherein, during this optimization treatment, an optimized value        of the height Hy which is to be used during a subsequent        scanning operation is determined as a function:    -   of at least one previously measured value of at least one        parameter representing the quality of the image acquired by the        sensor means,    -   and/or of at least one item of information issuing from a        previously executed decoding stage,        and wherein the sensor means are adapted to record the optimized        value of the height Hy determined in this way to be used during        a subsequent scanning operation.

In a device and a process according to this second variation of theinvention, the value of the height Hy, in particular the pitch hy and/orthe selection of the row(s) of height pyj used, is therefore optimizedafter each scanning operation to improve the results of the subsequentscanning stage thus enabling the decoding process to be executed andaccelerated and enabling the number of scanning stages required fordecoding to be reduced, with a field depth, electricity consumption androtational sensitivity round the optical axis which are compatible withpractical use of the device, and with an electronic scanning devicewhich is free from moving parts.

In particular in the case of simple bar codes or bar codes of the PDF417 type, the electronic processing means determine, after each scanningoperation, the value of the height Hy optimized to obtain the best fielddepth with given rotational sensitivity.

Advantageously and according to the invention, the optimized value ofthe height Hy is determined by computation, by closed loop control onthe basis of a reference value of a parameter or by optimization controlby comparing the evolution of at least one parameter of a readingoperation to another. Advantageously, therefore, automatic control isincorporated in the electronic processing means.

Advantageously and according to the invention, the optimized value ofthe height Hy is determined as a function of at least one previouslymeasured value, in particular after the previously effected last stageof scanning, of at least one parameter representing the quality of theimage acquired by the sensor means selected from the maximum spatialfrequency fx of the symbol in the scanning direction XX′, the maximumintensity of at least one category of symbol image elements, the minimumintensity of at least one category of symbol image elements and thecontrast of at least one category of symbol image elements. Furthersimilar parameters may be used as an alternative or in combination.

Advantageously and according to the invention, the optimized value ofthe height Hy is determined as a function of at least one item ofinformation relating to the type of symbol to be acquired and issuingfrom a previously executed decoding stage.

Advantageously and according to the invention, each previously measuredvalue and/or each item of information used to determine said optimizedvalue(s) has been obtained and recorded during a scanning operationimmediately preceding said optimization treatment.

Advantageously and according to the invention, the electronic processingmeans are adapted to fix by default and to record an initial value Hy°of the height Hy before a first scanning operation in order to acquire acode and/or after a last scanning operation in order to acquire a symbolor code, in particular a value hy° of the pitch hy. For example, hy°−h/2may be selected in which h is the total number of rows corresponding tothe total height H of the sensor in direction YY′.

Advantageously and according to the invention, the electronic processingmeans are adapted to, after each scanning operation:

-   -   determine the measured value of the maximum spatial frequency fx        of the symbol image in the scanning direction XX′,    -   calculate and record the optimized value of the height Hy on the        basis of an affine function of the inverse of the measured value        of the maximum spatial frequency fx of the code image in the        scanning direction XX′.

Advantageously and according to the invention, moreover, the electronicprocessing means are adapted to determine the optimized value of theheight Hy according to functions parameterized by predefined values, inparticular predefined by the user or during manufacture and stored in aread-only memory of the device, of parametric coefficients linked to thetype(s) of symbols to be acquired.

Advantageously and according to the invention, the electronic processingmeans are adapted to determine, after at least one scanning operation,in particular after a first scanning operation in order to acquire asymbol or after each scanning operation, the corresponding type ofsymbol and the value of the corresponding parametric coefficients. Forexample, if a number of gray levels higher than 2 is detected with acharacteristic homogeneous spatial frequency, it is probable that thesymbol is of the PDF 417 symbology type and therefore comprises aplurality of bar fines and that Hy was greater than the height of theimage of a line of bars of the symbol. It is thus possible to imposesubsequent criteria on the height Hy, in particular on the pitch try, inparticular that Hy is smaller than 4 times the width of the image of thefinest element of the symbol, this being a necessary condition foracquiring PDF 417 symbols.

Advantageously and according to the invention, the electronic processingmeans are adapted to calculate the optimized value of the height Hyaccording to a function parameterized by a predefined value of themaximum permitted angular deviation θmax of the sensor round the opticalaxis ZZ′ relative to the symbol to be acquired.

In an advantageous variation of the invention, the electronic processingmeans are adapted to determine and, if necessary, modify the optimizedvalue of the height Hy in order to optimize the measured value of thecontrast of at least one category of symbol image elements. To this end,the electronic processing means may include closed loop control adaptedto optimize the contrast.

In a further variation of the invention, the optimized value of theheight Hy is determined by computation.

Advantageously and according to the invention, the two-dimensionalsensor is a surface sensor formed by a CCD or APS matrix of pixels. Thissensor can have a plurality of embodiments.

In a first embodiment, the pixels in the same row are juxtaposed and arealigned in the scanning direction XX′, the sensor being formed by apixel matrix having h lines. In other words, each row of pixels isformed by one of the lines of the sensor. The pixels are generallysquare or rectangular.

In further embodiments, the electronic processing means and the sensorare adapted so that the pixels in the same row belong to two distinctlines of sensor pixels which are adjacent to one another in directionYY′, parallel to the scanning direction XX′, the successive pixels ineach row, when covering a row in the pixel-by-pixel scanning direction,alternately belonging to either of these two lines. This variationenable the dimension of the sensor in the scanning direction text to bereduced with the same resolution. The pixels may be square orrectangular, or may have other, generally polygonal, shapes.

Furthermore, the height ply of the pixels in each row may be the same(py) in all rows or, on the contrary, different from one row to another.

Advantageously and according to the invention, moreover, the opticalmeans comprise a diaphragm of which the dimensions are approximately theminimum dimensions corresponding to the theoretical limit of diffractionand are always greater than these minimum dimensions. Owing to theinvention, the smallest diaphragm allowed by the theoretical limit ofdiffraction may in fact be adopted. Advantageously and according to theinvention, therefore, the dimension 1 i of the diaphragm in thedirection II′ selected from the scanning direction XX′ or the directionYY′ is roughly but greater than:kλf(I+mi)/pi·Nminiwherein

-   -   λ is the wavelength of the lighting means,    -   f is the focal length of the optical means,    -   mi is the magnification of the optical means in the direction        II′,    -   pi is the dimension of the pixels of the sensor means in the        direction II′,    -   Nmini is the minimum number of successive pixels the direction        II′ which have to be contained in the image of a code element on        the sensor to allow the decoding thereof,    -   k is a form factor of the diaphragm.

Advantageously and according to the invention, an optimization treatmentcharacterized by at least one of the characteristics described above inrelation to the device according to the invention is executed.

It should be noted that U.S. Pat. No. 5,319,182 describes a mixed sensorwhich mixes light-emitting elements and light-sensitive elements whichcan be used in a bar code reader comprising a matrix of emitting andreceiving diodes, and aims to provide axial lighting for the targetaligned with the field of vision on the target to avoid the effects ofdiffusion and of layers. This document mentions configuration andoptimization of the grouping and proportion of emitters and detectors inthe matrix according to the image processing application thereof, inparticular for the reading of bar code symbols. However, this documentdoes not describe an electronic scanning device capable of adaptingitself to the unknown characteristics of a symbol to be read and inwhich the number of pixels in the direction perpendicular to thescanning direction is modified between two successive scanningoperations.

The invention also relates to a device and a process which arecharacterized in combination by all or some of the characteristicsmentioned hereinbefore or hereinafter.

Further objects, characteristics and advantages of the invention willemerge on reading the following description which refers to theaccompanying figures describing various non-limiting embodiments of theinvention.

BRIEF DESCRIPTION OF DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. The size and relative positions of elements in the drawings arenot necessarily drawn to scale. For example, the shapes of variouselements are not drawn to scale, and some of these elements arearbitrarily enlarged and positioned to improve drawing legibility.Further, the particular shapes of elements, as drawn are not intended toconvey any information regarding the actual shape of the particularelements, and have been solely selected for their ease and recognitionin the drawings.

FIG. 1 is a perspective view illustrating the geometry of a deviceaccording to the invention.

FIG. 2 is a schematic diagram illustrating an electronic processingcircuit of an optoelectronic device according to the invention.

FIG. 3 is a basic diagram in the image plane for determining a formulafor computation of the optimized value of the pitch hy in the case ofbar codes.

FIG. 4 is a flow chart of a first variation of a process according tothe invention.

FIG. 5 is A flow chart of a second variation of a process according tothe invention.

FIGS. 6 and 7 are diagrams illustrating two variations of a sensor of adevice according to the invention.

FIG. 8 is a graph showing an example of intensity signal obtained afterthe reading of a code.

FIG. 9 is a graph showing an example of intensity signal after thereading of the code following that in FIG. 8.

FIG. 10 is a graph showing a further example of an intensity signalobtained after the reading of a code.

FIG. 11 shows an example of an intensity signal spectrum obtained in thecase of a bar code having bars of two widths.

FIG. 12 is a diagram illustrating a further variation of a sensor of adevice according to the invention.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of theinvention. However, one skilled in the art will understand that theinvention may be practiced without these details. In other instances,well-known structures associated with machine-readable symbol readers,decoders, imaging devices, optics, computers, computer networks, datastructures, databases and networks, have not been described in detail toavoid unnecessarily obscuring the descriptions of the embodiments of theinvention.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprise” and variations thereof, such as“comprises” and “comprising” are to be construed in an open, inclusivesense, that is as “including but not limited to.”

FIG. 1 shows an optoelectronic device 34 according to the invention. Thegeneral characteristics of optoelectronic devices and their productionare well known. Therefore, only the novel characteristics of theoptoelectronic device will be described in detail.

This optoelectronic device 34 generally comprises a framework 35defining a reading Window 7 in front of which the symbols to be acquiredsuch as bar code symbol 1 are presented. The teachings apply to anybichromatic symbol such as bar codes, area codes, matrix or stackedcodes formed by elements of monochromatic geometric patterns having oneof two contrasting levels of color, of which the shapes and arrangementare adapted so that each symbol can biuniquely represent a value ofinformation to be acquired. The example in FIG. 1 is a traditionalmonodimensional bar code.

The framework 35 of the optoelectronic device 34 contains an opticalassembly 2, a sensor 3, and light source 4. The light source 4 can, forexample, take the form of an LED strip and its associated optics, forilluminating the bar code symbol 1. The bar code symbol 1 to be acquiredis placed opposite the reading window 7. The optoelectronic device 34may be of the portable type intended to be moved by a user in front of asymbol to be acquired. Alternatively, the optoelectronic device 34 maybe of the stationary type, the symbol itself being moved manually orotherwise in front of the reading window of the device.

The optical assembly 2 includes a diaphragm 5 and a set of one or morelenses 6 forming an optical system having an optical axis ZZ′ adapted toform an image of the bar code 1 symbol to be acquired on the sensor 3.

The sensor 3 takes the form of a two-dimensional sensor which is atleast substantially centered on the optical axis ZZ′ of the opticalassembly 2. The sensor 3 includes a plurality of individual lightingdetectors known as pixels. The pixels transmit electrical signalsrepresenting the quantity of light which they receive. The sensor 3 isof the electronic scanning type. In other words, the sensor's 3 pixelsare read successively, individually or in groups, one after the other ina global scanning direction, known as scanning direction XX′,perpendicular to the optical axis ZZ′. The sensor 3 is formed by amatrix of pixels ordered in a plurality of h rows, h being an integergreater than 1, juxtaposed in the direction YY′ perpendicular to thescanning direction XX′ and to the optical axis ZZ′. The scanningdirection XX′ corresponds to a direction of alignment or at least, tothe global direction of arrangement (median direction in the case ofpixels covered in a zig-zag manner) of the pixels of the sensor 3. Asensor 3 of this type is well known as such. Each line of pixels isconnected to an output, and the pixels are read successively byoffsetting according to the frequency of a clock signal controlling thesensor 3.

In the embodiment shown in FIGS. 1 and 2, the sensor 3 is a simplematrix of pixels ordered in straight lines and in straight columns. Thepixels in a same row are therefore juxtaposed and are aligned in thescanning direction XX′ corresponding to the direction of the lines ofthe sensor 3 thus formed by a matrix of pixels having h lines of pixelsall of the same size in the direction YY′, known as height py.

FIG. 6 shows an embodiment in which the pixels in the lines which areadjacent in direction YY′ are longitudinally offset, each row of thesensor 3 being formed by two adjacent lines of which the pixels may becovered alternately from one line to another in a zig-zag manner. Thevalue of such scanning is to reduce the dimension of the sensor 3 in thescanning direction XX′ while maintaining the same resolution. Thedrawback, however, is to increase the overall dimension of the sensor 3,in the direction

The embodiment shown in FIG. 7 is similar to that in FIG. 6, apart fromthe fact that the pixels are not square but hexagonal. Further similarvariations are obviously possible.

In the embodiment shown in FIG. 12, the sensor 3 comprises at least twoadjacent lines of pixels having different pixel heights pyj. Namely oneline of pixels of small height pyl and one line of pixels of largeheight py2. In the simplest embodiment, the sensor 3 comprises only twolines. However, there is nothing to prevent multiplication of the linesof different heights. It should be noted in this respect that the heightof the bar code symbol image on the sensor 3 is much greater than theheight of the individual lines of the sensor 3, and is also greater thanthe total height H of the sensor 3.

FIG. 2 shows an electronic processing circuit 36 of the optoelectronicdevice 34. All the pixels in each row of the sensor 3 are connected inseries to one of the output pins 8 of the sensor 3 which thereforecomprises h output pins 8. The output pins 8 are connected via a set ofswitches 9 to the input of an adding circuit 10. The adding circuit 10transmits a signal at an output 11 to an acquisition circuit 12 thatrecords the various values obtained at the output 11 over time, in otherwords during the scanning operation. The acquisition circuit 12transmits an intensity signal to a decoding logic circuit 13. Thedecoding logic circuit 13 supplies a trigger signal to a control logiccircuit 14, equipped with one or more microprocessors which controls theoperation of the sensor 3. In particular, the logic circuit 14 controlsa register 15 associated with the switches 9 such that the values of theregister 15 control the opening or closing of each of the switches 9placed between each output pin 8 of the sensor 3 and each correspondinginput of the adder 10.

It is shown schematically in FIG. 2 that, when a bit of the register 15is set to zero, the corresponding switch 9 is open whereas when this bitis set to 1, the corresponding switch 9 is closed. In the example shownin FIG. 2, the sensor 3 comprises eight rows and only the third row, thefourth row and the fifth row (numbered from the bottom) are connected tothe adding circuit 10. At each moment, the signals issuing from thesethree rows are therefore added at the output 11. During each scanningoperation, the hy rows of the sensor 3 which are active, in other wordsselected to form a portion, known as the scanned portion 16 of thesensor 3, are scanned simultaneously in parallel by means of the supplycircuit 7.

The electronic processing circuit 36 groups the pixels in groups ofadjacent pixels, known as individual read elements, during each scanningoperation. Each group of pixels has the same number of pixels, which isgreater than or equal to 1. The signals from all the pixels in the groupare added, and the resulting signal used for decoding the bar codesymbol. The various individual read elements extending successively inthe scanning direction XX′ while defining a selection of row(s) formingthe scanned portion 16, scanned by the scanning operation and having adimension in the YY′ direction, known as height Hy, which is the samefor all the individual read elements forming the portion scanned by asame scanning operation. Each individual read element comprises, in eachrow of the scanned portion 16, the same number hx of adjacent pixels inthe scanning direction XX′ and comprises a number hy of adjacent rows,in other words pixels, in the direction YY′.

In a first embodiment, the electronic processing circuit 36 is adapted,during each scanning operation, to activate an individual read elementat each moment and to cover the two-dimensional sensor 3 with successiveindividual read elements all having the same number of pixels definingthe selection of row(s) of the two-dimensional sensor 3 forming saidscanned portion 16, the individual read elements having a dimension,known as height Hy, in the direction YY′ which remains constant duringthe same scanning operation. Alternatively, the pixels may be groupednot electronically but logically on the basis of the signals receivedfrom each pixel individually.

During a scanning operation, each of the pixels of each of the rows isread according to the frequency transmitted by the control logic circuit14. The decoding logic circuit 13 is therefore adapted and programmed tocombine and add the successive pixels in the scanning direction XX′according to a pitch hx which, in the example illustrated, is equal to2. Thus, the pixels of the sensor 3 are grouped in successively readindividual read elements extending in the direction YY′ according to apitch hy, that is a height Hy=hy×py, and in the scanning direction XX′,according to a pitch hx, that is a width Hx=hx×px, px being the width ofeach pixel (dimension in the scanning direction XX′).

In the variation illustrated, the hy rows are combined in the directionYY′ by adding the analogue signals, whereas the hx pixels are combinedlogically by programming the decoding logic circuit 13. Furthervariations are possible. For example, the hx pixels may be grouped byreading according to an appropriate frequency with addition of theanalogue signals of the successively read pixels. It is also possible toscan all the lines but to select and group the hy rows logically byprogramming the programmable logic circuit 13.

In the case of the sensors 3 as illustrated in FIGS. 6 and 7, thecontrol logic circuit 14 is able to control, during each clock pulse, achange in the value of the register 15 in order to read the adjacentlines of a row alternately.

Alternatively in all cases, all the pixels of all the lines may be readindividually or successively and the programmable logic circuit 13 maybe programmed so as to group the hx×hy pixels and to add their signalsto obtain the individual read elements corresponding to the scannedportion 16.

In FIG. 2, the active useful area is indicated by hatching, this areacorresponding to the scanned portion 16 of the sensor 3 which allowscollection of the luminous intensity issuing from the optical means 2and used for reading the bar code symbol, during the subsequent decodingoperation. It should be noted that, here again, the scanned portion 16may be defined electronically, the sensor means receiving the signalsoriginating solely from this scanned portion or, alternatively, by logicmeans, the sensor 3 being read in its entirety but only the signalsissuing from the pixels of the scanned portion 16 then being exploited.At any moment, the scanned portion 16 of the sensor 3 corresponds to arectangle having hy rows in height and hx pixels in width correspondingto an individual read element. All the electrical signals issuing fromthe pixels of this individual read element are added at the output 11,so, the greater the number of pixels of this individual read element hxand hy, the greater the signal obtained with the same luminous intensityreceived on the sensor 3.

According to the invention, the microprocessor equipped control logicmeans 14 incorporate a logic processing program allowing the value ofthe height Hy of the scanned portion 16 to be optimized, in particularthe value of the pitch hy of this scanned portion 16.

FIG. 4 shows a first variation of a process according to the inventionemployed in a device according to the invention to allow automaticdetermination of the optimized value of the height Hy, in particular ofthe pitch hy, in order to optimize the field depth of the deviceaccording to the invention. During stage 17, the values of the height Hyand of the width Hx of the individual read elements, in particular thevalues of the pitches hx and hy, are initialized to predeterminedinitial values Hy° and Hx°, in particular hy° and hy°. In practice,these initial values are of little importance in so far as the processaccording to the invention is converging very quickly. For examples, hy°and hx° may be fixed at 1. Alternatively, hy°=h/2 (or ENT (h/2) if h isodd) and hx°=1 may be selected, if monodimensional bar codes are to beacquired a priori. In the case of PDF 417 type bar codes, hy°=1 andhx°=1 may be selected. Whatever the type of code to be acquired, theprocess according to the invention will allow decoding to be effectedmore or less rapidly depending on the initial value hy° implemented. Inthe variation in FIG. 12, at least one of the rows is selected, in otherwords at least one of the heights pyj.

It should be noted, however, that in view of the current clocking ratesof the microprocessors which may be used in the control logic means 14,the scanning and decoding rate is extremely fast and, in any case, muchfaster than the relative displacement of the bar code symbol 1 withrespect to the sensor 3, which may be induced by the movements of ahuman operator.

The subsequent stage 18 corresponds to scanning for the purpose ofreading of the bar code symbol 1, in other words for carrying outcomplete scanning of the sensor 3, with the previously recorded andselected value of the height Hy, in particular of the pitch hy and/or ofthe height pyj of the row(s), of the scanned portion 16, the lightsource 4 being active. The subsequent stage 19 corresponds to theexecution of the decoding protocol. This decoding protocol is well knownas such and consists, on the one hand, in determining the type ofmachine-readable symbol to be acquired, then, depending on the type ofsymbol to be determined, the values of information represented by thesymbol. During the subsequent stage 20, it is determined whether or notthe decoding process 19 has ended with the total decoding of the barcode symbol 1. If so, the process is terminated during the final stage21, then returns to the start of the initialization stage 17 in order toacquire a new bar code symbol 1.

If the stage 20 determines that decoding has not ended, logic processingis executed in stage 22 to determine whether or not the height Hy, inparticular the pitch hy and/or the selection of the row(s), of thescanned portion 16 should be modified. A new optimized value of thisheight Hy is determined and recorded during a subsequent modificationstage 23. After determining the new optimize value, a return is made tothe beginning of stage 18 in order to carry out a new scanning operationand a new reading of the bar code symbol 1 to be acquired. In thisvariation, therefore, the decoding process 19 is carried out after eachstage of scanning 18. During the logic processing stage 22, at least onemeasured value of at least one parameter representing the quality of theimage acquired by the sensor 3 during the scanning stage 18 or during anearlier scanning stage 18, or one or more items of information issuingfrom an earlier decoding stage 19, or both at least one such measuredvalue and at least one such item of information can therefore be takeninto consideration.

The parameters representing the quality of the image acquired by thesensor 3 which may be selected include, in particular, the maximumspatial frequency fx of the symbol image in the scanning direction XX′and/or the maximum intensity Imax of the symbol image and/or the minimumintensity Imin of the symbol image and/or the contrast of at least onecategory of elements of the symbol image or of the contrast values ofthe various elements of the symbol image. The contrast of a symbol or ofa category of elements of a symbol (for example all the bars of the samewidth) may be represented by the value (Imax−Imin)/(Imax+Imin) obtainedfor the entire symbol image or for a category of elements of the symbolimage. Other contrast formula may be used.

The optimized value of the height Hy, in particular of the pitch hyand/or of the height(s) pyj, may be calculated and determined on thebasis of the measured value of at least one of these parameters duringthe reading stage 18. For example, if it is known that the symbols to beacquired are monodimensional bar codes with one line, a predeterminedformula may be used. Advantageously and according to the invention,therefore, the electronic processing circuit 36, is adapted to calculatethe optimized value of the pitch hy in the direction YY′ for a symbol ofthe monodimensional bar code type having a line corresponding to theformula (I):hy=ENT [(1/(py·tan(θmax)))·[(1/(2·fx)−(Nminx·hx·px))]  (I)

-   -   wherein, θmax is the maximum permitted angular deviation round        the optical axis of the sensor means relative to the symbol to        be acquired,    -   px is the dimension of the pixels in the scanning direction xx′,    -   fx is the maximum spatial frequency of a previously read image        of the symbol in the scanning direction XX′,    -   py is the dimension of the pixels in direction YY′,    -   Nminx is the minimum number of groups of successive pixels in        the scanning direction XX′ which have to be contained in the        image of a symbol element on the sensor to allow the de coding        thereof,    -   ENT is the total part function,    -   and wherein hx=ENT [1/(2fx·Nminx·px)].

This formula (I) is obtained with the construction in FIG. 3 whichshows, in the image plane, a portion of the sensor 3 and two bars 31, 32of the bar code symbol 1 separated by the smallest distance(corresponding to the maximum spatial frequency of the symbol). In thisdiagram, it has been assumed that hx=1, Nminx=2 and hy=8.

Similarly, if it is known that the symbols to be acquired are PDF 417type codes (pluri-monodimensional bar code symbols having a plurality oflines juxtaposed in the vertical direction), a predetermined formula maybe used. Advantageously and according to the invention, therefore, theelectronic processing circuit 36 is adapted to calculate the optimizedvalue of the pitch hy in direction YY′ for a bar code symbol of the typeknown as PPF 417 corresponding to formula (II):hy=ENT{MIN[(Ky/(2fx·py),[(1/py·tan(θmax)))·[1/(2·fx)−(Nminx·hx·px)]]}  (II)

-   -   wherein, Omax is the maximum permitted angular deviation round        the optical axis of the sensor means relative to the symbol to        be acquired,    -   px is the dimension of the pixels in the scanning direction XX′,    -   fx is the maximum spatial frequency of a previously read image        of the symbol in the scanning direction XX′,    -   py is the dimension of the pixels in direction YY′,    -   Nminx is the minimum number of groups of successive pixels in        the scanning direction XX′ which have to be contained in the        image of a symbol element on the sensor to allow the decoding        thereof,    -   ENT is the total part function,    -   Ky is an integer determined to allow decoding of the PDF 417 bar        code symbols,    -   MIN is the minimum function,    -   and wherein hx=ENT [1/(2fx·Nminex·px)].

More generally, however, the invention allows the value of Hy to beoptimized without even knowing a priori the type of symbol to beacquired. The type of symbol may be sought during the stage 22 of logicprocessing by the electronic processing circuit 36 after the firstreading 18 of a symbol to be acquired. For example, if the output signalobtained has, as shown in FIG. 9, a number of levels of distinctintensity greater than 2 with a characteristic homogeneous spatialfrequency, it is certain that a plurality of different elements of thesymbol are covered by the pitch hy in direction YY′. Consequently, thesymbol cannot be formed by a monodimensional bar code. A PDF 417 typesymbol, in particular, is therefore involved. Now with this type ofsymbol there is a condition whereby Hy≦Ky·mx,Ixmin,namely≦Ky·mx·Ixmin/py.

This condition may therefore be taken into consideration whendetermining the new value of Hy, in particular of py. Even moregenerally, the value of Hy can be optimized without even knowing anddetermining the type of symbol to be acquired.

FIG. 8 shows an example of intensity signal 37 which may be obtained atthe output 11 of the adder 10 after a first scanning 18 of a bar codesymbol having bars of two widths. This signal 37 enables the bars ofgreater width to be detected between their maximum intensities IImax andminimum intensities IImin, but attenuates the bars of small width. Thisattenuation may be due either to defocusing of the image or to anexcessive value of Hy. After such a signal 37, Hy may therefore bereduced, for example by a unit, to carry out a new scanning operation.If it is found that the signal has not improved after this new scanningoperation, in other words that the bars of smaller width are notdetected, the problem originates from defocusing which has to be treatedby a different solution. On the other hand, if the problem originatesfrom the value of Hy, the signal 37 will be improved as shown in FIG. 9,where the bars of smaller width appear with their maximum intensityI2max and minimum intensity I2min. This modification of Hy can thereforebe pursued, if the signal to noise ratio is sufficient, until decodingis achieved.

FIGS. 10 and 11 show a further example of information on the type ofsymbol which may be obtained after the reading stage 18. If the contrastis sufficient, in a monodimensional bar code symbol 1, which may havebars of two different widths, the signal spectrum obtained is normallyclassified by two values fx₁ and fx₂ of the spatial frequency in thedirection XX′. If the presence of these two values is effectively noted,it is known that the symbol to be acquired is of the type having bars oftwo distinct widths, and a preferential value for Hy can therefore bededuced therefrom. For example, the aforementioned formula (I) can thenbe applied in order to determine hy for a monodimensional bar codesymbol.

Dotted lines 38 (FIG. 11), illustrate the trend of the spectrum in thecase where the contrast is not sufficient to discriminate between thetwo frequencies fx₁ and fx₂. In this case, it may nevertheless beconsidered that the maximum spatial frequency fx of the code image inthe scanning direction XX′ has a median value fx₃ which may be used incalculating the pitch hy.

FIG. 5 shows a variation of the process in which a treatment fordetermining and modifying the height Hy, in particular the pitch hyand/or the height(s) pyj of the selected row, according to the measuredvalue of at least one parameter relating to the quality of thepreviously obtained image, is carried out independently of a treatmentemploying at least one item of information issuing from the decodingstage 19. In this variation, the decoding process is not in fact carriedout after each scanning operation. This variation therefore alsocomprises an initialization stage 17, a scanning stage 18 for reading asymbol to be acquired, immediately followed by a stage 24 for measuringat least one parameter representing the quality of the image asmentioned hereinbefore. During the subsequent stage 25, it is determinedwhether or not the criteria relating to each of these parameters aresatisfied. In other words, it is determined whether or not the imagequality is satisfactory for carrying out a decoding stage. From amongthese criteria, for example, it may be required that the contrast of atleast one category of elements of the symbol image (for example the barsof small width) is greater than a predetermined value, or that themaximum spatial frequency fx of the symbol image in the direction XX′,is within a predetermined maximum type deviation. If these criteria arenot satisfied, the decoding stage 19 is not carried out. Instead, alogic processing stage 22 a is carried out which determines (bycomputation or optimization) whether or not the value of the height Hyhas to be modified. The process then loops, executing the stage 23 ofmodification, recording the new optimized value of Hy, and returning toa new scanning stage 18. Thus, successive scanning stages are carriedout and the value Hy is modified if the quality of the image is notadequate, in the sense of the criteria predetermined during stage 25.

If the image quality is considered sufficient during stage 25, thedecoding stage 19 is carried out. Stage 20 follows, which determineswhether or not decoding has been successful. If decoding has beensuccessful, the process is ended during stage 21. If not, a logicprocessing stage 22 b is carried out, which determines, on the basis ofthe information coming from the decoding stage 19, whether the value ofthe height Hy should be modified. The new value Hy is recorded, ifapplicable, during the modification stage 23, and the processreiterates, executing a new reading operation 18.

To calculate the optimized value of the height Hy of the scanned portion16, an optimized value of the pitch hy (number of rows) can becalculated and/or, in the variation in FIG. 12, the selection ofadjacent row(s) having a total height closest to the optimum value canbe determined, for example by selecting the row having the mostappropriate height pyj.

The process according to the invention may be carried out by programmingon the basis of the above-described logic functions.

The invention may form the subject of numerous variations. Inparticular, numerous distinct optimization treatments may be employeddepending, in particular, on the types of symbols to be acquired. Moregenerally, all the known digital optimization processes and digital oreven analog automatic controls are applicable (proportional regulator,derivative, integral, PID, etc.).

Furthermore, instead of optimization of Hy, it may be sufficient tostore various predetermined values of Hy or to calculate thesepredetermined values according to a formula independent of the symboltype (which depends only on a serial number of the scanning stage to becarried out). Then all the various possible scanning operations arecarried out, each with one of these values. In the variation in FIG. 12,the various scanning operations may even be carried out with each of therows or with each possible selection of row(s). The decoding protocol iscarried out after each scanning operation or after all scanningoperations. If the decoding protocol is carried out after each scanningoperation, the successive scanning/decoding operations may be continuedat least until the machine-readable symbol is recognized, by modifyingHy between two successive scanning operations. If the decoding protocolis carried out after all the scanning operations, the results obtainedduring the scanning operations can be sorted by quality (for example bycontrast and/or intensity) and decoding can be commenced with the bestresults.

Although specific embodiments, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications can be made without departing from the spirit and scope ofthe invention, as will be recognized by those skilled in the relevantart. The teachings provided herein of the invention can be applied toother systems for reading machine-readable symbols, not necessarily thebar code symbol reading system generally described above. The variousembodiments described above can be combined to provide furtherembodiments. The illustrated methods can omit some acts, can add otheracts, and can execute the acts in a different other than thatillustrated to achieve the advantages of the invention. The teachings ofthe applications, patents and publications referred to herein, areincorporated by reference in their entirety.

These and other changes can be made to the invention in light of theabove detailed description. In general, in the following claims, theterms used should not be construed to limit the invention to thespecific embodiments disclosed in the specification, but should beconstrued to include all imaging systems that operate in accordance withthe claims. Accordingly, the invention is not limited by the disclosure,but instead its scope is to be determined entirely by the followingclaims.

1. An optoelectronic device for acquiring machine-readable symbols,comprising: a sensor array comprising a plurality of light sensingelements, the light sensing elements producing a signal representative aquantity of light incident on the respective light sensing element; anda scanning control circuit coupled to selectively sample the respectivesignals from the light sensing elements of the sensor array and operableto change a resolution of the sensor array in a direction perpendicularto a scanning direction, between at least a first resolution during atleast a first sampling pass and a second resolution during at least asecond sampling pass, the second sampling pass following the firstsampling pass, wherein the sensor array is a two-dimensional array oflight sensing elements arranged in a plurality of rows, the lightsensing elements in each of the rows extending in the scanningdirection, and the plurality of rows arranged in the directionperpendicular to the scanning direction with respect to one another, thelight sensing elements in a first one of the rows having a first heightmeasured in the direction perpendicular to the scanning direction, andthe light sensing elements in a second one of the rows having a secondheight measured in the direction perpendicular to the scanningdirection, the second height different from the first height.
 2. Theoptoelectronic device of claim 1 wherein the scanning control circuitselectively samples the signals from the light sensing elements in thefirst row during the first pass and selectively samples the signals fromthe light sensing elements in the second row during the second pass. 3.An optoelectronic device for acquiring machine-readable symbols,comprising: a sensor array comprising a plurality of light sensingelements, the light sensing elements producing a signal representative aquantity of light incident on the respective light sensing element; anda scanning control circuit coupled to selectively sample the respectivesignals from the light sensing elements of the sensor array and operableto change a resolution of the sensor array in a direction perpendicularto a scanning direction, between at least a first resolution during atLeast a first sampling pass and a second resolution during at least asecond sampling pass, the second sampling pass following the firstsampling pass, wherein the scanning control circuit during the firstpass selectively samples signals from the light sensing elements in afirst pair of rows having a first cumulative height in the directionperpendicular to the scanning direction, and during the second passselectively samples signals from the light sensing elements in a secondpair of rows having a second cumulative height in the directionperpendicular to the scanning direction, different than the firstcumulative height.
 4. An optoelectronic device for acquiringmachine-readable symbols, comprising: a sensor array comprising aplurality of light sensing elements, the light sensing elementsproducing a signal representative a quantity of light incident on therespective light sensing element; and a scanning control circuit coupledto selectively sample the respective signals from the light sensingelements of the sensor array and operable to change a resolution of thesensor array in a direction perpendicular to a scanning direction,between at least a first resolution during at least a first samplingpass and a second resolution during at least a second sampling pass, thesecond sampling pass following the first sampling pass, wherein thescanning control circuit during the first pass selectively samplessignals from the light sensing elements in a first number of rows havinga cumulative first height in the direction perpendicular to the scanningdirection, and during the second pass selectively samples signals fromthe light sensing elements in a second number of rows having acumulative second height in the direction perpendicular to the scanningdirection, the cumulative second height different than the cumulativefirst height.
 5. An optoelectronic device for acquiring machine-readablesymbols, comprising: a sensor array comprising a plurality of lightsensing elements, the light sensing elements producing a signalrepresentative a quantity of light incident on the respective lightsensing element; and a scanning control circuit coupled to selectivelysample the respective signals from the light sensing elements of thesensor array and operable to change a resolution of the sensor array ina direction perpendicular to a scanning direction, between at least afirst resolution during at least a first sampling pass and a secondresolution during at least a second sampling pass, the second samplingpass following the first sampling pass, wherein the sensor array is atwo-dimensional array of light sensing elements arranged in a pluralityof rows, the light sensing elements in each of the rows extending in thescanning direction, and the plurality of rows arranged in the directionperpendicular to the scanning direction with respect to one another, thelight sensing elements in adjacent rows being offset from one another inthe scanning direction.
 6. The optoelectronic device of claim 5 whereinthe light sensing elements are sampled in order generally along thescanning direction and alternating between immediately adjacent ones ofthe rows.
 7. A method of operating an optoelectronic device to acquiremachine-readable symbols, the optoelectronic device including a sensorarray comprising a plurality of light sensing elements, each of lightsensing elements producing a signal representative of a quantity oflight incident on the respective light sensing element, the methodcomprising: receiving signals from a first set of the light sensingelements at a first resolution during a first sampling pass across thesensor array while an image of a first machine-readable symbol is formedon the sensor array; and receiving signals from a second set of lightsensing elements at a second resolution during a second sampling passacross the sensor array while the image of the first machine-readablesymbol is formed on the sensor array, the second resolution differentfrom the first resolution in a direction perpendicular to the first andsecond sampling passes, wherein the sensor array is a two-dimensionalarray of light sensing elements arranged in a plurality of rows, thelight sensing elements in each of the rows extending in a scanningdirection, and the plurality of rows arranged in a directionperpendicular to the scanning direction with respect to one another, thelight sensing elements in a first one of the rows having a first heightmeasured in the direction perpendicular to the scanning direction, andthe light sensing element in a second one of the rows having a secondheight measured in the direction perpendicular to the scanningdirection, the second height different from the first height, andwherein receiving signals from a first set of the light sensing elementsat a first resolution during a first sampling pass comprises selectivelysampling the signals from the light sensing elements in the first rowduring the first pass and wherein receiving signals from a second set oflight sensing elements at a second resolution during a second samplingpass comprises selectively sampling the signals from the light sensingelements in the second row during the second pass.
 8. A method ofoperating an optoelectronic device to acquire machine-readable symbols,the optoelectronic device including a sensor array comprising aplurality of light sensing elements, each of light sensing elementsproducing a signal representative of a quantity of light incident on therespective light sensing element, the method comprising: receivingsignals from a first set of the light sensing elements at a firstresolution during a first sampling pass across the sensor array while animage of a first machine-readable symbol is formed on the sensor array;and receiving signals from a second set of light sensing elements at asecond resolution during a second sampling pass across the sensor arraywhile the image of the first machine-readable symbol is formed on thesensor array, the second resolution different from the first resolutionin a direction perpendicular to the first and second sampling passes,wherein receiving signals from a first set of the light sensing elementsat a first resolution during a first sampling pass comprises selectivelysampling signals from the light sensing elements in a first pair of rowshaving a first cumulative height in a direction perpendicular to ascanning direction, and wherein receiving signals from a second set oflight sensing elements at a second resolution during a second samplingpass comprises selectively sampling signals from the light sensingelements in a second pair of rows having a second cumulative height inthe direction perpendicular to the scanning direction, the secondcumulative height different than the first cumulative height.
 9. Amethod of operating an optoelectronic device to acquire machine-readablesymbols, the optoelectronic device including a sensor array comprising aplurality of light sensing elements, each of light sensing elementsproducing a signal representative of a quantity of light incident on therespective light sensing element, the method comprising: receivingsignals from a first set of the light sensing elements at a firstresolution during a first sampling pass across the sensor array while animage of a first machine-readable symbol is formed on the sensor array;and receiving signals from a second set of light sensing elements at asecond resolution during a second sampling pass across the sensor arraywhile the image of the first machine-readable symbol is formed on thesensor array, the second resolution different from the first resolutionin a direction perpendicular to the first and second sampling passes,wherein receiving signals from a first set of the light sensing elementsat a first resolution during a first sampling pass comprises selectivelysampling signals from the light sensing elements in a first number ofrows having a first cumulative height in a direction perpendicular to ascanning direction, and wherein receiving signals from a second set oflight sensing elements at a second resolution during a second samplingpass comprises selectively sampling signals from the light sensingelements in a second number of rows having a second cumulative height inthe direction perpendicular to the scanning direction, the secondcumulative height different than the first cumulative height.
 10. Amethod of operating an optoelectronic device to acquire machine-readablesymbols, the optoelectronic device including a sensor array comprising aplurality of light sensing elements, each of light sensing elementsproducing a signal representative of a quantity of light incident on therespective light sensing element, the method comprising: receivingsignals from a first set of the light sensing elements at a firstresolution during a first sampling pass across the sensor array while animage of a first machine-readable symbol is formed on the sensor array;and receiving signals from a second set of light sensing elements at asecond resolution during a second sampling pass across the sensor arraywhile the image of the first machine-readable symbol is formed on thesensor array, the second resolution different from the first resolutionin a direction perpendicular to the first and second sampling passes,wherein the sensor array is a two-dimensional array of light sensingelements arranged in a plurality of rows, the light sensing elements ineach of the rows extending in a scanning direction, and the plurality ofrows arranged in a direction perpendicular to the scanning directionwith respect to one another, the light sensing elements in adjacent rowsbeing offset from one another in the scanning direction, and wherein thelight sensing elements are sampled in order generally along a scanningdirection, alternating between immediately adjacent rows of the sensorarray.
 11. A method of operating an optoelectronic device to acquiremachine-readable symbols, the optoelectronic device including a sensorarray comprising a plurality of light sensing elements, each of lightsensing elements producing a signal representative of a quantity oflight incident on the respective light sensing element, the methodcomprising: receiving signals from a first set of the light sensingelements at a first resolution during a first sampling pass across thesensor array while an image of a first machine-readable symbol is formedon the sensor array; receiving signals from a second set of lightsensing elements at a second resolution during a second sampling passacross the sensor array while the image of the first machine-readablesymbol is formed on the sensor array, the second resolution differentfrom the first resolution in a direction perpendicular to the first andsecond sampling passes; and determining a new height for a set of thelight sensing elements after each sampling pass based on at least onepreviously measured value of at least one parameter representing aquality of an image of a symbol acquired by the optoelectronic device,the quality selected from: a maximum spatial frequency of the image in ascanning direction, a maximum intensity of at least one category ofsymbol element in the image, a minimum intensity of at least onecategory of symbol element in the image, and a contrast of at least onecategory of symbol element in the image.